Process for the production of hard burnt pellets having good abrasive strength



United States Patent 3,338,705 PROCESS FOR THE PRODUCTION OF HARD BURNTPELLETS HAVING GOOD ABRASIVE STRENGTH Kurt Meyer, Frankfurt am Main,Georg von Struve, Neu Isenburg, Guenther Reimann, Oberursel, Tauuus, andKarl-Heinz Boss, Bad Homburg, Germany, assignors to MetallgesellschaftAktiengesellschaft, Frankfurt am Main, Germany No Drawing. Continuationof application Ser. No. 397,279, Sept. 17, 1964. This application Mar.23, 1967, Ser. No. 625,556

2 Claims. (Cl. 753) This application is a continuation of applicationSer. No. 397,279 filed Sept. 17, 1964, now abandoned.

The invention relates to a process for the production of hard-burntpellets from iron ores and iron ore concentrates the fine grain contentof which is less than 0.044 mm. (minus 325 mesh) and insufiicient toform pellets having the necessary abrasive strength.

The abrasive strength is one of the most important strength factors ofhard-burnt iron pellets because it determines whether and how thepellets withstand the treatment after hard-burning. In practice it isgenerally determined by the ASTM drum test. Sutficient abrasive strengthis present when the pellets having an average diameter of 12 mms. do notshow more than 6 to 8% fines with a grain size of less than 0.6 mm. (28mesh) after an 8 minute continuous tumbling test in a standard ASTM cokebreeze drum rotating at 25 revolutions per minute.

For forming and producing hard-burnt pellets possessing suflicientabrasive strength, a finess of the initial ore was hitherto considerednecessary in which 80 to 90% of the total quantity of ore was less than0.044 mm. (minus 325 mesh). It was also already pointed out that whengrinding the ore to a content of only 60% less than 0.044 mm., the wholeof. the ore must be ground until all grain sizes are less than 0.2 mm.in diameter in order to be able to ball the ore.

In the US. Patent 2,960,396, particularly lines 49 et seq. of column 1,the determination of the ballability of iron ores by the percentage ofparticles less than 0.044 mm., is described as a relatively rough methodand it is stated that the content of fine grain material which is lessthan 0.01 mm. is the determining factor for the ballability of the ironore and, to a certain extent, also for the strength of the hard-burntpellets formed thereform. Nevertheless this method which is described asrough is still employed in practice, which is explained by the fact thatwork in mining installations and steel mills is naturally rough.Furthermore, the rougher method is more economical because as a rule itis more tedious, complicated and expensive to grind a small portion ofthe ore by itself, that is separately from the whole quantity of ore tobe pelletized, to a considerable percentage in minus 0.01 mm., and thento mix it again in suitable quantities with the ore to be pelletized,than continuously to bring the whole of the ore to a certain content offine grain in minus 0.044 mm. by rougher methods, whereby at any rate,the effective fine content less than 0.01 mm. just also be present insufiicient quantity.

While a few years ago there was still a prejudice in the technical worldagainst the production of iron ore pellets from ore mixtures in whichthe individual components possess dilferent chemical and/or physicalproperties, the opinion in the technical world has changed recently inthat it is admitted that it is possible to produce such mixed orepellets also without disadvantage. However, the only advantages wereconsidered to be that the ores could now be pelletized in steel mills,harbours and the like, where with economical storage and preparation,the mixing of different kinds of ores is practical.

It has now been found that the production of hardburnt pellets with highabrasive strength from iron ores, iron ore concentrates and the like,using as initial material a mixture of ore components having differentchemical and/or physical properties, considerable advantages over thestate of the art can be achieved if, for forming the pellets, oremixtures are used in which the quantity of fine-grained material whichis less than 0.044 mm. amounts to only 20 or up to about 60% of thetotal quantity of ore and such mixtures are hard-burnt at temperaturesof at least 1200 C. By mixing different ores and burning the pelletsproduced therefrom at the temperatures mentioned, not only abrasivestrengths are obtained which are to be expected, taking intoconsideration the characteristics behaviour of the individual componentsof the mixture, but the pellets obtained according to the invention haveconsiderably higher abrasive strengths. Furthermore, it was found thatthese higher abrasive strengths are also obtained when the content offine-grained material less than 0.044 mm. is less than would beconsidered necessary for obtaining suflicient abrasive strength in thecase of the known state of the art. This presents the advantage thatores of coarser consistency need not be ground to the same extent whichwas considered necessary in the case of the known state of the art, withthe result that the pelletization is considerably simplified and lessexpensive. In such cases a quantity of fine grained material less than0.044 mm. amounting to 45% of the mixture is sufficient and as a rule itis not necessary to go beyond about 60% of such fine grained material.

The fine-grained content can be regulated in any suitable manner. Forexample, by mixing different kinds of iron ore with suitable fine graincontent or by suitable partial or complete grinding of the ore mixture.In any case it is not necessary to employ uneconomical means for thepurpose, such as, for example, tedious, expensive grinding of the wholequantity of one of the components to a grain size less than 0.07 mm.

Grinding the whole of the ore to the fineness mentioned 1 has been foundmost advantageous, especially as a very thorough mixing of the ores isgenerally obtained thereby.

Particularly good pellets with high abrasive strength are obtainedaccording to the invention if a mixture of as many as possible dilferentkinds of ore components, that is a mixture of more than two components,is used as initial material. By so doing not only increased abrasivestrength but also improvement of other properties, such as for examplethe ballability of the initial material, increase of the compressionstrength of the hard-burnt pellets and so forth, are effected. Of greatimportance for producing the above-mentioned technical eifect is the useof sufllciently high burning temperatures. Actually temperatures of 1200C. and higher are known for the hard-burning of iron pellets. However, awhole series of cases are mentioned in literature and known in practicein which other temperatures of, for example 1100 or 0 C, are proposed oremployed in the case of certain ores and for the production of pelletsfor specific purposes.

An increase of the abrasive strength is not possible at these othertemperatures even in the case of ore mixtures. The process according tothe invention and the technical effect attainable thereby arehereinafter explained with the aid of practical examples.

3 Example 1 A mixture of the following ores was pelletized:

(a) Mano River (b) Bong Range (c) Nimba (d) Itabira (e) Carol LakeConcentrate.

The mixture had the following composition:

(a) 10% (b) 40% (c) 10% .(d) 20% (e) 20% Its chemical composition was:

Percent Fe 65.5 Si 3. 34 CaO 0.34 MgO 0.41 A1 0 1.06

Before pelletization the ores or the mixture were or was wet ground to afineness of 60% less than 0.044 mm. in a ball mill. Subsequently thewater was removed on a drum filter until the moisture content was suchthat subsequent pelletization was possible. In all cases 2% limestonewas added, as can be assumed to be known.

The pellets were produced on a plate 0.8 m. in diameter. Their averagediameter was 12 mm. The pellets were burnt on a pan, about 56 to 59 kgs.being charged at a time. The burning temperature amounted to 1,300 or1,350 C.

The quality of the pellets was tested by the ASTM Drum T est generallyused today.

The following abrasive strengths were obtained for the single orepellets and mixed ore pellets respectively:

(e) not worked into pellets with sufiicien-t dry strength Mixture: 4.8%

The tests showed clearly that the mixture produced pellets having abetter abrasive strength than the pellets of the individual components.

Example 2 As in the case of Example 1, pellets to 15 mm. in diameterwere produced. For this purpose a magnetite concentrate and a minetteore mixture were used, having the following compositions:

The burning temperature was between 1250 and 1300 C. No binding agentswere used. While the minette ore mixture was too coarse to enablepellets of useful quality to be produced therefrom alone, pellets with7.7% minus 0.6 mm. could be produced from the magnetite concentrate. Ifthe two components were mixed in a ratio of 2:1, whereby there was 41%minus 0.044 mm., the abrasive strength of the burnt pellets amounted to6.1%. In the case of a mixture in a ratio of 1:1 and 20.0% fines minus0.044 mm., the abrasive strength of the burnt pellets was 6.5%, which isalso higher than could be attained in the case of pelletization of theindividual concentrates.

We claim:

1. Process for the production of hard-burned pellets having highabrasive strength from iron ores, iron ore concentrates and the like,and using an initial material composed of a mixture of ores havingdifferent chemical and/or physical properties, comprising grinding allthe ore components together to a fine grain material in which from 20 tohas a fineness of less than 0.044 mm., forming moist rounded pellets ofsaid material, and then hard burning the pellets at a temperature of atleast 1200 C.

2. Process as in claim 1, said initial material composed of the mixtureof ores being composed of more than two kinds of iron ore.

References Cited UNITED STATES PATENTS 728,369 5/1903 Chase -3 1,129,6452/1915 Collard 75-3 2,131,006 9/1938 Dean 75-3 2,960,396 11/1960 DeVaney75-3 3,189,436 6/1965 Burstlein 75-3 3,244,507 4/ 1966 Linney 75-33,254,985 6/1966 Merklin 75-3 BENJAMIN HENKIN, Primary Examiner.

1. PROCESS FOR THE PRODUCTION OF HARD-BURNED PELLETS HAVING HIGHABRASIVE STRENGTH FROM IRON ORES, IRON ORE CONCENTRATES AND THE LIKE,AND USING AN INITIAL MATERIAL COMPOSED OF A MIXTURE OF ORES HAVINGDIFFERENT CHEMICAL AND/OR PHYSICAL PROPERTIES, COMPRISING GRINDING ALLTHE ORE COMPONENTS TOGETHER TO A FINE GRAIN MATERIAL IN WHICH FROM 20 TO60% HAS A FINENESS OF LESS THAN 0.044 MM., FORMING MOIST ROUNDED PELLETSOF SAID MATERIAL, AND THEN HARD BURNING THE PELLETS AT A TEMPERATURE OFAT LEAST 1200*C.